Metal detectors have become increasingly important for recreational, industrial, and security applications. A typical metal detector includes a resonant circuit that is responsive to electrical signal losses in the resonant circuit associated with metal objects situated near the resonant circuit. As the resonant circuit and objects in a region of interest are moved with respect to each other, an electrical signal is produced that is associated with any conductive objects in the region of interest. Sensitive detection of these electrical signals is possible with compact circuit elements so that a typical metal detector can be made portable and readily configured for various applications.
Unfortunately, conventional metal detectors exhibit significant drawbacks that limit their applications. Electrical signals associated with the presence of metal objects tend to be very small, and are frequently mixed with substantial amounts of random or other noise. Thus, detection of small metal objects is difficult or impossible. In addition, some noise sources associated with the environment in which metal detectors operate can mask the presence of metal objects. For example, temperature changes can produce noise signals that are much larger than signals associated with metal detection, and false detection alarms can be produced as a result. As a result of these significant amounts of random noise and environmental noise, some objects of interest cannot be detected or are associated with impractically high false positive detection rates. Accordingly, improved metal detection methods and apparatus are needed.